Physical Aging Effects on Molecular-Scale Polymer Relaxations Monitored with Mobility-Sensitive Fluorescent Molecules

The fluorescence response of a mobility- or free volume-sensitive molecule, julolidenemalononitrile (JMN), is shown to be affected by the relaxations that occur on the molecular scale during physical aging in various glassy polymers including polystyrene, polycarbonate, poly(methyl methacrylate), and poly-(isobutyl methacrylate). In general, the change in fluorescence intensity is approximately linear with logarithmic aging time as has been found in bulk volume and enthalpy relaxations. Near the glass transition temperature, JMN monitors the relaxations until equilibrium is reached where the fluorescence response maintains a constant value. Additionally, the physical aging rate was found to depend on temperature in all polymers as has been observed in bulk specific volume behavior. The temperature trends found in polystyrene, poly-(methyl methacrylate), and polycarbonate with the molecular fluorescence technique agree well with those from specific volume relaxation. These results provide experimental evidence of how the molecular environments contribute to the traditionally observed bulk properties.